This invention relates generally to a process comprising coinjecting or coextruding a structural polymer resin with one or more performance polymer resins to a form a multilayer article without melt flow defects.
Poly(ethylene terephthalate) (PET) is an established bottle polymer that produces rigid bottles with excellent clarity and gloss. These containers are manufactured by a process that comprises drying the PET resin, injection molding a preform and, finally, stretch blow molding the finished bottle.
The injection molding of PET preforms requires the melting of polymer pellets and the injection of the molten, viscous PET material into a cavity, which also has a core rod. The molten PET forms a xe2x80x9cskinxe2x80x9d where it comes into contact with the cold cavity wall and core rod. This skin is composed of xe2x80x9cfrozenxe2x80x9d PET and will remain fairly stationary throughout the remainder of the injection molding process.
At points extending radially inwardly away from the cavity wall and, outwardly from the core rod, or at the points at which the polymer does not directly contact the cavity wall or core rod, the polymer (which is still elevated in temperature) remains a viscous, flowing mass. This hot inner viscous material can still flow relative to the frozen skin layer although its viscosity increases as it continues to cool. Thus, a temperature transition region occurs in the radial direction as well as a corresponding melt viscosity transition (because of PET""s viscosity dependence upon temperature). Regardless of the changes in melt viscosity as a function of radial distance from the skin, monolayer PET is, for the most part, unaffected by the shear that develops between the frozen skin of the PET and the molten polymer that pushes past it. After the entire cavity has been filled using this process, the polymer is held in the cavity until the preform has become sufficiently cool so that it can be blown immediately into a bottle or the preform is cool enough to be ejected. Cooled preforms that have been ejected are stored for later reheat blow molding into the final product.
Using this process, PET resin is used in a wide range of applications such as carbonated soft drink, hot-filled juice products and warm-filled foods. However, PET has insufficient barrier to meet the desired shelf lives of products with more demanding gas barrier needs.
In one particular application, in order to increase the gas barrier of a PET bottle, it is possible to inject a barrier layer into or onto a preform during the injection molding process. This barrier layer is injected into or onto the melt flow stream of the PET such that the barrier polymer resin flows past the skin of PET previously injected. This xe2x80x9ccoinjectionxe2x80x9d process allows two resins to be injected into a xe2x80x9cmultilayerxe2x80x9d preform that can be blown to form the final bottle product.
Unfortunately, it has been found that the coinjection of a barrier polymer resin with PET can result in defects in the PET preform. A commonly observed melt flow defect is small xe2x80x9cpulls,xe2x80x9d frequently called chevrons because of their V shape. Chevrons are interfacial instabilities that occur between layers. Chevrons detract from the aesthetics of the finished article.
One barrier resin that may be used in a multilayer process is an ethylene-vinyl acetate copolymer (EVOH) modified with various levels of ethylene (xe2x80x9cgradesxe2x80x9d). It is commonly known that these xe2x80x9cgradesxe2x80x9d of barrier resins have different melt viscosities and melting points. Generally, it would be desirable to match both the melt viscosity of the barrier resin and the melt temperature of the barrier resin to the PET being used. Unfortunately, the commercially available EVOH (regardless of the grade) has a melt viscosity and degradation temperature far below that of commercially available PET. In addition, heat transfer from the hotter PET layer will further heat the EVOH above its desired processing temperature and result in even lower melt viscosity of the barrier resin during injection molding.
Most of the technology for coinjection is relatively new and is just becoming commercially viable for molding multilayer articles or preforms on a large scale. In addition, coinjection for most practical purposes is focused almost solely on the use of PET (or a copolymer thereof) as the structural resin for preform molding applications. In contrast, coextrusion is a well-established technique that is commonly applied to a wide variety of different polymers (e.g., PET, copolyesters, polyolefins, PVC, styrenics, nylons, etc.) and for a much wider range of applications.
In coextrusion, multilayer film or sheet is produced as opposed to a molded article. As with coinjection, there is one or more xe2x80x9cstructuralxe2x80x9d layers combined with one or more xe2x80x9cperformancexe2x80x9d layers. The structural layers are usually (but not always) cheaper than the performance layers and are included to keep total cost down (since performance layers can often be expensive). Examples of coextrusion include the use of a barrier layer in packaging film, the use of a UV protecting layer on the outside layer of heavy gauge sheeting for outdoor weathering protection, the use of regrind in the center to reduce costs, the use of adhesive/sealing layers on the outside surface, and the use of glossy and/or pigmented layers to change the overall aesthetics of the film/sheet. Unlike the coinjection example cited above, the xe2x80x9cperformancexe2x80x9d layer in coextrusion does not necessarily have to be on the inside of the multilayer structure.
In the process of coextrusion, the various resins are first melted in separate extruders and then brought together in a feedblock-a feedblock being nothing more than a series of flow channels which bring the layers together into a uniform stream. From this feedblock, this multilayer material then flows through an adapter and out a film die. The film die can be a traditional flat film/sheet die (e.g., a coathanger die) or it can be an annular die as is used in blown film. Coextrusion is also used making more complicated shapes like profiles. When we refer to coextrusion in this document, it is implied that all of these other coextrusion applications are also covered in addition to traditional film/sheet applications.
As with coinjection, coextrusion often suffers with the problem of chevrons and other visual defects. These defects in coextrusion and coinjection both result from high shear stresses developing at the layer interface during flow. These stresses are a function of the viscosities of the layers in addition to the relative position and thickness of the layers. In fact, knowledge gained from coextrusion can be used to help minimize the flow defects in coinjection.
In addition, coextrusion of flat film often suffers from the problem of poor layer distribution across the width of the sheet. For example, if one were to take a piece of coextruded film (for example, an A/B/A structure) and separate the layers, they might find that one of the A layers would be much thicker near the outer edges of the sheet, and very thin in the middle. The B layer would be just the opposite, that is, being thin near the edges and thick in the middle. Usually, it is desired that the layers be uniform in thickness across the full width of the sheet so that properties (e.g., barrier, color, stiffness, etc.) do not vary across the width.
Up until now, correcting these two coextrusion problems (poor layer distribution uniformity and flow defects) has really been more of an art than science. There have been some attempts to balance the viscosities of the resins (i.e., having a viscosity ratio close to one) to improve layer distribution, but this has met with only limited success. Thus, there exists a need for a process to properly select both the resin viscosity and elasticity parameters and the processing conditions in coextrusion such that both the interfacial instabilities (i.e., visual defects like chevrons) and poor layer distribution are eliminated.
In the coextrusion process according to this invention, therefore, the xe2x80x9celasticityxe2x80x9d of the various resin layers is as important as the resin viscosity and proper balancing of both the elasticity ratio and the viscosity ratio simultaneously is needed in order to have a uniform layer distribution and form a multilayer article. A process has thus been developed so that processing conditions and resins can be to optimized to eliminate these multilayer flow problems.
Because the multilayer flow behavior is very similar for both coinjection and coextrusion, the method can be effectively applied for both applications. As a result, the process of the present invention forms a high quality coinjected multilayer article or preform as easily as it forms a multilayer coextruded film structure.
The present invention relates to the elimination of melt flow defects such as chevrons from coinjected and/or coextruded articles by minimizing the interfacial stress between layers, such as between a structural layer (e.g., PET) and a performance (e.g., barrier) layer, in a multilayer molded structure or article.
In addition, the present invention relates to the matching of viscoelastic flow properties of the respective layers so that layer distribution is maintained in a uniform fashion for coextrusion and coinjection applications.
As embodied and broadly described herein, this invention, in one embodiment, relates to a process for coinjection-molding a multilayer article. The process comprises coinjecting at a selected coinjecting temperature (i) a first outer polymer resin layer having a viscosity at the selected coinjecting temperature, and (ii) a second inner polymer resin layer having a viscosity at the selected coinjecting temperature, wherein the ratio of the outer polymer resin viscosity to the inner polymer resin viscosity at the coinjecting temperature is less than or equal to about 2 and the coinjecting temperature is above the melting temperature of the highest melting resin and below the degradation temperature of the lowest degrading resin to form a multilayer article.
In another embodiment, the present invention comprises a process for coextruding a multilayer article comprising coextruding at a selected coextruding temperature (i) a first outer polymer resin layer having a viscosity at the selected coextruding temperature, and (ii) a second inner polymer resin layer having a viscosity at the selected coextruding temperature, wherein the ratio of the outer polymer resin viscosity to the inner polymer resin viscosity at the coextruding temperature is less than or equal to about 2 and the coextruding temperature is above the melting temperature of the highest melting resin and below the degradation temperature of the lowest degrading resin.
In another embodiment, the present invention relates to a process for coinjection-molding a 5-layer article comprising coinjection-molding at a selected coinjecting temperature (i) two outer polymer resin layers having a viscosity at the selected coinjecting temperature, (ii) two intermediate polymer resin layers disposed between a core layer and the two outer layers, the two intermediate resin layers having a viscosity at the selected coinjecting temperature, and (iii) a core layer having a viscosity at the selected coinjecting temperature, wherein at each polymer resin interface the ratio of the outermost polymer resin viscosity to the next innermost polymer resin viscosity at the coinjecting temperature is less than or equal to about 2 and the coinjecting temperature is above the melting temperature of the highest melting resin and below the degradation temperature of the lowest degrading resin to form a 5-layer article.
In yet another embodiment, the present invention relates to a process for coextruding a 5-layer article comprising coextruding at a selected coextruding temperature (i) two outer polymer resin layers having a viscosity at the selected coextruding temperature, (ii) two intermediate polymer resin layers disposed between a core layer and the two outer layers, the two intermediate resin layers having a viscosity at the selected coextruding temperature, and (iii) a core layer having a viscosity at the selected coextruding temperature, wherein at each polymer resin interface the ratio of the outermost polymer resin viscosity to the next innermost polymer resin viscosity at the coextruding temperature is less than or equal to about 2 and the coextruding temperature is above the melting temperature of the highest melting resin and below the degradation temperature of the lowest degrading resin to form a 5-layer article.
Additional advantages of the invention will be set forth in part in the detailed description, including the figures, which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory of preferred embodiments of the invention, and are not restrictive of the invention, as claimed.